1. Technical Field
The invention relates to an electro-acoustical devices and, more particularly, to a loudspeaker for use in constructing a line array of loudspeakers.
2. Description of the Problem
Large space, public sound systems rely on a combination of loudspeaker types to achieve efficiency, wavefront coherence, a broad and level audio frequency bandwidth and good coverage of an audience located in the space. A foundational element in many such public sound systems is a line array of multi-transducer loudspeakers. A line array, in its classical form, consisted of a vertical row of “closely spaced”, cone type, direct radiator acoustical drivers set in a baffle. In this arrangement adjacent acoustical drivers are mutually coupled to reduce the spread of the sound in the plane comprising the axis of alignment of the drivers and to promote even diffusion of the sound energy in an expanding half cylinder having the axis of alignment as its center. Since line arrays are typically oriented vertically, this means that sound reproduced has a minimal vertical spread and enhanced horizontal audio qualities and exhibit greater efficiency.
Mutual coupling of the acoustic drivers results from the acoustic drivers being identical, producing the same sounds and being closely spaced. What constitutes “closely spaced” is a function of the highest audio frequency that the array is intended to produce, but roughly means that the center of each speaker cone should be spaced from adjacent cones by no more than a quarter wavelength of the highest frequency sound the array is intended to reproduce. Audible sound ranges in wavelengths from about 17 meters at 20 hertz to 1.7 cm at 20 Kilohertz. The smallest direct radiator speakers used are usually on the order 10 cm. allowing sound reproduction up to a frequency of about 3 Kilohertz. This provides for good speech intelligibility but is less suitable for amplification of music into large spaces.
The prior art exhibits repeated attempts to provide line arrays capable of reproducing the highest discernable frequencies by constructing devices which emulate certain characteristics of ribbon or “planar” type devices. Ribbon type audio transducers date back to the early twentieth century. An example of an early ribbon type audio transducer is disclosed in U.S. Pat. No. 1,809,754 for an “Electrostatic Reproducer” to Steedle. Ribbon devices resemble an elongated flat panel and produce sound from a vibrating flat surface. In effect a ribbon or planar is a line array of infinitesimal elements positioned directly adjacent one another, i.e. a line array having zero spacing between mutually coupled drivers. This in turn means that a planar has no practical upper frequency limit in the human audio range. Unfortunately, as observed by Adamson in U.S. Pat. No. 6,343,133, ribbon tweeters have had limits in sensitivity and power handling capacity preventing application of the devices in replacing high frequency compression drivers in systems for large spaces. Planars can also suffer from selective frequency cancellation due to out of phase reflection issues from the mounting enclosure used in linear array units.
Due in part to the perceived problems with planars, several attempts have been made to produce a device that behaves like a planer but is constructed using horn loaded, conventional mid or high frequency drivers. Such a device is usually intended to be used to reproduce sound over a broad frequency range. Precursors to and examples of these devices are represented by U.S. Pat. No. 4,344,504 to Howze, U.S. Pat. No. 5,163,167 to Heil, U.S. Pat. No. 6,343,133 to Adamson, and U.S. Pat. No. 6,394,223 to Lehman. The proposed systems have obtained some of the performance, high frequency fidelity and efficiency gains of a planar. However, horn loading introduces some distortion to sound reproduction. It would be desirable to produce a loudspeaker which overcomes the problems with incorporating ribbon devices used in line arrays and which minimizes the need to resort to horn loading to situations requiring greater output power than is currently the case.